Compound semiconductor nanowires have the potential for being the fundamental building blocks of our future high-speed and high-efficiency electronics. In particular, transistors based on compound semiconductors such as GaAs or InAs have properties far superior to that of their silicon-based counterparts, promising for example a tremendous boost of clock frequency of future processor chips. However, the high lattice-mismatch between GaAs and Si so far prevents conventional approaches relying on planar semiconductor heterostructures. Vertical semiconductor nanowires, in contrast, provide an elegant solution to this problem, since any lattice mismatch can be accommodated by relaxation processes via the nanowire sidewalls. A major step towards large scale application of semiconductor nanowires in electronics is their epitaxial integration into the well-established silicon platform.
Vapor-liquid-solid mode is the most widely used approach for growing GaAs nanowires on silicon. This approach utilizes a liquid metal droplet as a catalyst and a material reservoir for axial nanowire growth. Variants are the Au-catalyzed method and the self-catalyzed method where the liquid metal droplet consists of Ga. The self-catalyzed method has considerable advantages compared to the use of foreign metals as catalyst, avoiding any problems with contaminations and further processing. While control of the crystal structure has effectively been achieved in Au-catalyzed GaAs nanowires, control of the nanowire shape and crystal structure still present significant challenges in the case of highly desirable self-catalyzed VLS-grown nanowires.
The size of the droplet used defines the actual diameter of the nanowire, while the microstructure of the nanowire is determined by the shape of the droplet, and especially by the wetting conditions at the vapor-liquid-solid interface. Certain wetting conditions facilitate the nucleation of different GaAs crystal phases whose coexistence is called polytypism. The hexagonal wurtzite phase of GaAs (usually metastable in the bulk) grows at moderate wetting angles (90°), and the cubic zinc blend phase generally at larger wetting angles (130° ). The transition between the two phases is usually associated with faulted regions connected to the transient behavior of the liquid Ga-droplet.
The droplet also affects the radial growth, especially for large wetting angles, resulting in tapering, that is, a variation of the nanowire diameter along its length. Therefore, tapered nanowire geometries can be explored to understand on the one hand the radial growth processes that are responsible for their formation and on the other hand the impact of the liquid Ga-droplet, in order to gain better control of the overall growth process. Moreover, control over tapering would allow new perspectives for crystal engineering, for example of axial nanowire heterostructures.
A group of researchers at the University of Siegen in Germany: Dr. Philipp Schroth, Seyed Mohammad Mostafavi Kashani, Jonas Vogel and Professor Ullrich Pietsch in collaboration with Julian Jakob, Dr. Ludwig Feigl and Professor Tilo Baumbach from Karlsruhe Institute of Technology (KIT), Dr. Jörg Strempfer and Dr. Thomas Keller from Deutsches Elektronen-Synchrotron (DESY), all in Germany, have investigated the evolution of shape and crystal structure of inversely tapered self-catalyzed GaAs nanowires. They have utilized time-resolved in situ X-ray diffraction during the growth, together with scanning electron microscopy for ex situ post-growth characterization, interpreting their results using the diameter self-stabilization model. Their research work has been published in the research journal, Nano Letters.
By following the evolution of polytypism and nanowire radius in their experiment, the authors were able to distinguish the side-wall growth and tapering for the self-catalyzed GaAs nanowires as well as the radial growth processes responsible for each of them.
As a significant aspect of their work, the authors successfully refined the existing theoretical growth models for the diameter self-stabilization process, thereby fully understanding the shape evolution of liquid Ga-droplet during the growth process.
The authors are optimistic that their experimental approach will advance the understanding of VLS growth processes, especially for the synthesis of phase-pure nanowires, and provides a method for evaluating and refining currently existing nanowires growth models.
Schroth, P., Jakob, J., Feigl, L., Mostafavi Kashani, S., Vogel, J., & Strempfer, J. Keller, T. F., Pietsch, U., & Baumbach, T.(2018). Radial Growth of Self-Catalyzed GaAs Nanowires and the Evolution of the Liquid Ga-Droplet Studied by Time-Resolved in Situ X-ray Diffraction. Nano Letters, 18(1), 101-108.Go To Nano Letters